1. Field of the Invention
The present invention relates to a reception apparatus and method of a Multiple Input Multiple Output (MIMO) system that receives a plurality of different data streams in a multiple cell environment.
This work was supported by the IT R&D program of MIC/IITA [2006-S-002-02, “IMT-Advanced Radio Transmission Technology with Low Mobility”].
2. Description of Related Art
Optimum Combining (OC) technology is used in designing a wireless communication system in a multi-cell environment to remove cochannel interference and thereby prevent deterioration in performance caused by the cochannel interference (CCI). As the field of wireless communication services extends from conventional low-speed voice communication to high-speed multimedia communications, such technologies as Singular Value Decomposition (SVD) and Vertical Bell Lab Space Time (V-BLAST) are used to increase. data transmission rate.
Optimum Combining technology removes interference signals in a cellular-based multiple access system employing Multiple Input Multiple Output (MIMO) antennas. The technology prevents performance deterioration by removing interference between channels, i.e., cochannel interference. Also, the technology can maximize signal-to-noise ratio (SNR) by reducing influence of fading.
Optimum combining technology is what Maximal Ratio Combining (MRC) technology is extended into an interference-existing environment. Signals are regarded as colored noise added thereto in the environment where there is interference. The interference signal can be removed by using a whitening filter to thereby turn the colored noise into white noise and combining outputs of the whitening filter through Maximal Ratio Combining. Maximal ratio combining is a technology for maximizing an output signal-to-noise ratio in an MIMO antenna system, and it independently gives a weight to each data stream received in each antenna of an MIMO antenna reception apparatus. Generally, a weight maximizing the signal-to-noise ratio of data streams is selected. Data streams are combined according to the given weight to thereby remove cochannel interference.
Also, in order to remove cochannel interference and decrease the influence of fading, the MIMO antenna system suggests a Smart Base and Smart Mobile (SBM) antenna structure. Multiple antennas of a transmission apparatus are given with directivity based on a beamforming vector, individually, and transmit data streams in a desired direction. Herein, each data stream transmitted through the antennas of the transmission apparatus functions as interference onto data streams transmitted through the other antennas, the reception apparatus removes the cochannel interference and maximizes signal-to-interference plus noise ratio (SINR) by using an Optimum Combining onto the data streams received through multiple antennas.
This method increases signal reception performance and acquires diversity gain because each antenna combines data streams that have undergone different Rayleigh fading. However, when an antenna transmits and receives a plurality of different data streams based on beamforming and optimum combining technology, interference may be added to the combination of the different data streams, which leads to performance deterioration. Therefore, conventional MIMO antenna systems employing beamforming and optimum combining technology are designed on the assumption that the same data streams are received. The conventional MIMO antenna systems may increase transmission liability but it does not reach a transmission rates required for high-speed wireless communication. In short, the conventional MIMO antenna systems have a problem in that they cannot acquire multiplexing gain.
FIG. 1 illustrates a typical antenna system based on MIMO Optimum Combining (MIMO-OC) technology. The drawing shows a 4×4 MIMO antenna system 100 including a transmission apparatus 100 and a reception apparatus, each having four antennas 120 and 140.
The transmission apparatus 110 simultaneously transmits a plurality of data streams (a1x1) through beamforming. Herein, the data streams (a1x1) are all the same. Since a plurality of data streams pass through diverse channel paths, diversity gains are acquired as much as a multiplication of the numbers of transmission and reception antennas. Therefore, the antenna system 100 of FIG. 1 can acquire 4×4 antenna gains.
The reception apparatus 130 receives a plurality of data streams (a1x1) through multiple reception antennas 140. As described before, when a plurality of bit streams are transmitted through multiple transmission antennas 120, data stream received by each antenna acts as interference onto data streams received by the other antennas. Therefore, the reception antenna 140 removes cochannel interference by using optimum combining onto the data streams received by the multiple antennas.
However, when a plurality of different data steams (a1x1, a2x2, a3x3 and a4x4) are transmitted through each of the transmission antennas, both cochannel interference and interference between bit streams occur at the same time. Thus, the reception apparatus 130 cannot combine the different data streams. When this problem arises, conventional technology acquires diversity gain by increasing the number of transmission antennas 120 to thereby improve liability. However, when a plurality of different bit streams are transmitted simultaneously, it cannot performing the combining and thus it cannot acquire multiplexing gain.
As a solution to this problem, Vertical Bell Lab Space Time (V-BLAST) technology independently transmits data streams from multiple transmission antennas to reach a channel capacity of an MIMO antenna system, which is a data transmission rate required for high-speed wireless communication. V-BLAST is a less complex version of existing Diagonal Bell Lab Space Time (D-BLAST) structure. The V-BLAST technology simply demultiplexes serial data streams to be transmitted into parallel data streams according to the number of antennas of the transmission apparatus. The parallel data streams are simultaneously transmitted through the transmission antennas, respectively, to thereby increase the channel capacity.
Also, when both transmission apparatus and reception apparatus are aware of channel information, a waterfilling allocation method of a power based on singular value decomposition (SVD) may be used. With singular value decomposition, a channel matrix may be decomposed into two unitary matrixes and a diagonal matrix having its eigenvalue in a diagonal term. A channel comes to have a diagonal form without interference between streams when the unitary matrixes obtained from the decomposition are multiplied by the channel information of the reception and transmission apparatuses. Also, when an inverse number of the eigenvalue the diagonal term, which can be regarded as a gain of each channel, is allocated as a transmission power, the MIMO antenna system can reach the desired channel capacity.
The conventional technology increases channel capacity and acquires multiplexing gain by raising a data transmission rate in such a manner that different data streams that are independent from each other are transmitted through the antennas of the transmission apparatus. However, the conventional technology has a shortcoming that cochannel interference cannot be removed. In other words, the conventional technology cannot acquire diversity gain. Removal of cochannel interference requires a combining structure in the reception apparatus. However, the singular value decomposition technology and the V-BLAST technology described before cannot use the combining technology because they transmit independent data streams from all transmission antennas. After all, all the antennas of the reception apparatus are used to remove the interference between the independent data streams, the convention technologies cannot prevent performance deterioration cause by cochannel interference.
Therefore, it is required to develop an MIMO antenna system that can secure high data transmission rates while minimizing the influence of cochannel interference in a multi-cell environment.